The present invention relates generally to ultrasound. In particular, the present invention relates to motion adaptive frame averaging for color flow imaging in an ultrasound device.
Some known modes of diagnostic ultrasound imaging include B- and M-modes (used to image internal, physical structure), Doppler, and color flow (primarily used to image flow characteristics, in blood vessels for example). Ultrasound color flow mode is typically used, for example, to detect the velocity of blood flow toward/away from a transducer, essentially utilizing the same technique used in Doppler mode. Whereas Doppler mode imaging displays velocity versus time for a single selected sample volume, ultrasound color flow mode imaging displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity.
Using Doppler effects to measure blood flow in the heart and blood vessels, for example, is known. The amplitude of the reflected waves may be employed to produce black and white images of the moving tissue, while the frequency shift of backscattered waves may be used to measure the velocity of the backscatterers from tissue or blood. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer.
Color flow ultrasound imaging may be used to provide a vivid display of both blood flow and basic anatomy. Color flow images may be produced by superimposing a color image of the flow velocity of the moving material, blood or tissue for example, over the gray scale B-mode image. The color pixels represent the flow velocity of the moving material. Frame averaging may be applied to the color pixel displays to improve signal to noise ratio (alternatively referred to as “SNR”) and persistency. Frame averaging uses a newly acquired color frame and a sequence of previously acquired color frames to determine the current color frame to be displayed to the end user. Various infinite impulse response (alternatively referred to as “IIR”) and finite impulse response (alternatively referred to as “FIR”) filter techniques may be applied to create a display color frame using a sequence of consecutive color frames in time. Arithmetic mean operation is a simple and commonly used FIR filter techniques that may be used for calculating the display frames from the sequence of frames acquired.
One limitation associated with frame averaging in color flow ultrasound imaging is that it creates a significant amount of lag and smearing due to motion, where the motion may be caused by probe or tissue movement. Such lag and smearing of color pixels is very prominent to the user, as it often overlaps the gray scale stationary tissue background from the B-mode display. The End users often choose to reduce the level of frame averaging to avoid the lag and smearing alias. However, in reducing the level of frame averaging, the end user may lose any advantages associated with higher SNR and persistency provided by frame averaging.